In hard X-ray optics monitoring an intensity of hard X-rays (photon energies ˜5-50 keV) incident on an optical element typically requires a stand-alone X-ray detector placed upstream of the optical element. Such an X-ray detector can alter the incident X-ray beam by absorbing a fraction of incident radiation and/or disturbing the radiation wavefront. Furthermore, delicate detector electronics and operating conditions may not be always compatible with the harsh radiation environment of a synchrotron or XFEL beamline. In particular, it is problematic or in some cases impossible to monitor intensity of an intense X-ray beam incident on front-end beamline optics (e.g., primary X-ray windows, high heat load monochromators, X-ray mirrors and refractive lenses etc.). X-ray optical elements for hard X-rays are made of solid state materials such as Si, Ge, C (diamond), Be, SiO2 (quartz, silica), Al2O3 (sapphire) as well as metallic films (Pt, Au, and Pd) deposited on various substrates. A device that performs functions of an X-ray optical element and an X-ray monitor simultaneously should be compared primarily with solid state X-ray detectors. Solid state detectors are based on semiconductors (primarily Si and Ge). Detection of X-rays or some other types of electromagnetic radiation (such as visible light) requires tailoring of bulk semiconductor properties such as forming p-n junctions. The radiation incident on the active region of a detector then produces electric carriers which results in a measurable voltage or electric current. Such modification of a solid state material is generally not compatible with the performance characteristics of the optical element. For example, early demonstrations of X-ray detection by voltage developed across an optical element included a p-n junction within the diffracting Si crystal (i.e., required doping of Si to tune its electric properties). However, the best material for diffracting X-ray crystal optics is a high-purity (i.e., undoped) Si due to better crystal quality.
In recent years considerable effort has been made towards development of diamond solid state X-ray detectors which led to commercially available products such as beam position monitors and solid state ion chambers. Diamond is a particularly important material for solid state X-ray detector applications in harsh radiation environments due to its low X-ray absorption, high thermal conductivity and high radiation hardness. On the other hand, diamond is an electrical insulator characterized by absence of free carriers and a far-from-ideal dielectric with deep level traps in the band gap. Electrons and holes generated by absorption of X-ray photons remain trapped inside the crystal unless a penetrating electric field is applied. Most efforts for developing diamond and other solid state detectors for X-rays has been directed towards optimization of charge collection from the bulk of the material. Solutions such as application of bias voltage and reduction of bulk impurity concentration (diamond fabrication using chemical vapor deposition method (CVD)) have been implemented to mitigate poor bulk charge collection in diamond.
State-of-the-art CVD diamond based detectors have useful characteristics. However, most such detectors are delicate stand alone devices with limitations on radiation environments and can also disturb the radiation wavefront by presence of defects in the crystal structure of CVD diamond. These defects can also limit applicability of CVD diamond as an X-ray optical element (e.g. diffracting crystal). Although some robust diamond radiation detectors have been patented recently that may be compatible with hostile radiation environments, none have been claimed to perform a function of an X-ray optical element at the same time, except the most trivial one—an X-ray window. Therefore, there remains a substantial need for a simple but highly effective hard X-ray detector for X-ray optical systems.
In the soft X-ray regime (photon energies below 5 keV) detection of X-rays is often performed using total electron yield due to photoemission. Photoemission is one of the basic outcomes of interaction of X-rays with matter where an absorbed X-ray photon creates multiple photoionization events while some of the generated electrons leave the exposed material. The material (usually conductive) is in direct contact with a conductive sample holder that is connected to the electrical ground through a current meter. As an uncompensated charge develops due to escape of photoelectrons a compensating electric current flows to the substrate and is registered by the current meter. The magnitude of this current serves as a measure of the incident or absorbed photon flux. A similar strategy can be applied to detection of hard X-rays by a variety of X-ray optical elements, which, however does not require bulk conductivity in the materials.